1. Field of the Invention
The present invention relates to the art of enhancement of images, and more particularly to a method of and an apparatus for enhancing the sharpness of images generated by scanners in platemaking applications, for example.
2. Detailed Description of the Related Art
Sharpness enhancement has heretofore been employed for improving the sharpness of images in the field of image processing.
According to the conventional sharpness enhancement process, as well known in the art, an image signal is extracted from around an image signal S, whose sharpness is to be enhanced, of a subject (which may be an image-carrying document), with a mask size of n.times.n pixels, for example, and an arithmetic mean of the extracted image signal is calculated as an unsharp signal U (also called a "local mean signal"). Then, a difference signal S-U (also called an "unsharp masking (USM) signal")between the image signal S and unsharp signal U is calculated, and multiplied by a predetermined coefficient k which normally has a value in the range of from 0 to 1, generating an unsharp masking signal USM.
An image signal (also called a "sharpness-enhanced signal") S* which is sharpness-enhanced is produced according to the following equation (1): EQU S*=S+USM=S+k.multidot.(S-U) (1)
The sharpness-enhanced signal S* is then converted into a binary signal, which is converted into a halftone dot image signal. Based on the halftone dot image signal, a halftone dot image is formed on a recording medium such as a film by a scanning process. A printing plate is made on the basis of the recording medium, and then mounted on a printing press to produce a print with halftone dot image formed thereon.
When the above conventional sharpness enhancement process is effected on an image signal, a uniform density region of an image formed from the sharpness-enhanced signal, i.e., an image region where the image density is constant or varies gradually, is enhanced to a granular state. For example, an image on a print which is produced as described above may have a greater surface roughness.
Furthermore, when the above conventional sharpness enhancement process is effected on an image signal, quantized steps of an image signal in a region where the image density varies gradually are enhanced, resulting in stripes like tone jumps on an image on a print which is produced as described above.
One solution would be not to effect the sharpness enhancement process on an image region where the image density varies to the same extent as quantized steps. However, such an approach would be disadvantageous in that a visual surface characteristic, i.e., the texture, of a subject such as a fabric, a skin, a metal, or the like, would also be enhanced, and the image on the print would lose the texture of the subject.